The molybdenum coordination sites of the molybdoenzymes appear to fall into two classes, depending on the nature of the molybdenum-containing cofactor: the nitrogenases possess molybdenum bound to some type of Mo, Fe, S cluster, while it appears reasonably certain that Mo(VI), (V), and (IV)-oxo species are involved at the active sites of the molybdenum oxidases and nitrate reductase. In all cases, the molybdenum is coordinated to some type of sulfur-donor ligand. The major objective of the research is to explore the chemical and structural possibilities of molybdenum-sulfur complexes and to develop the electrochemical and reactivity trends exhibited by these species. The work in progress is concerned with the synthesis and chemical and structural characterization of complexes with Mo(IV), Mo(V), and Mo(VI) with polydentate, mercapto-containing ligands as models for the oxidases and as starting materials for the synthesis of molybdenum-hydrazido and diazenido-complexes. The Mo-hydrazido complexes provide models for the interaction of Mo in nitrogenase with a known enzyme substrate. The extensive structural and electrochemical investigations of these unusual species demonstrate that the Mo coordination number and oxidation state and the nature of the coligands contribute to determine the course of protic degradation reactions of the metal-bound hydrazine. Extensions of these studies to mixed-metal oligomers suggest that Mo-Fe clusters incorporating nitrogenase ligands may be synthesized and structurally characterized.